improved docs on summary table

[nick-youngblut]

The README doesn't seem to explain the scores in the summary table (abricate --summary), and I can't really tell if it's percent identity, percent coverage, or something else based on the code https://github.com/tseemann/abricate/blob/master/bin/abricate. It would be helpful to make it clear what those values are

[tseemann]

Nick, you have a very good point.
I'm not even sure what they are myself.
According to the code it is %COVERAGE.
I will improve the docs.
Thank you!

[tseemann]

Now in https://github.com/tseemann/abricate#combining-reports-across-samples